Bencent Electronics Power Port fire risk and solutions

Time:2017-06-26 09:39:18 Source: Nums：75

INTRODUCTION

In recent years, with the booming of lightning protection
components and defense scheme, power port will rarely be damaged by the lightning
stroke. However, the other problem is the fire accident that the voltage
dependent resistor is subjected to abnormal power frequency overvoltage.

The circuit applications risk of three in one power port

The below picture is the classical three in one
circuit of power port, three voltage dependent resistor plus one gas discharge
tube.：

According to the equipment application environment, if the power
port needs to tolerate a 5KA lightning stroke, then at least a 15A fuse is
needed in circuit. The working current of a 15A fuse is 30A. That is to say the
fuse cannot take a protective effect before 30A, but below 30A, the voltage
dependent resistor will be fired.

Video 1: added 1000V to the loop, the fuse of
20A and 15A could not work and the voltage-sensitive fired.

The applications risk of T-MOV

The standard of T-MOV is UL1449, the specific terms of the tripping
performance of T-MOV are as follows：

Permanently connected to the SPD, the maximum current
for the voltage test is 10A, so there is still a blind spot in the circuit.
When the current in the circuit is 10A-30A, T-MOV burns.

Video 2: added 1000V to the loop, the
voltage-sensitive fired when it was 20A.

An effective solution

To solve this problem completely, we need an ideal electronic component
with the following properties:

1. The lightning
strike can be passed normally, and the operating frequency (1-50a) shows the
open circuit failure;

2. The time of the
cut-off circuit is defined by different frequency current. Under this current
time, pressure sensitivity can be tolerated

3. Since the
impedance is uncertain after the pressure-sensitive breakdown, the heat is not
controlled, so it can\'t rely on the pressure sensitive heat to cut the circuit.

Fail-safe GDT can meet the three points into action (performance,
and can be quickly cut off the circuit, the power frequency approached gas
discharge tube below the graph is open circuit failure of power frequency
electric current and disconnection time test curve

Video 3: added 1000V to the loop, the Fail-safe
GDT cut off the circuit in 0.68s when it was 20A.

Video 4: added 1000V to the loop, the Fail-safe
GDT cut off the circuit in 0.52s when it was 40A.

It is determined by the internal structure of the Fail-safe GDT
which can quickly break the start frequency current without relying on external
heat.

Conclusion

General AC power supply three plus one solution and T - MOV scheme
still have a blind point to cause a fire. However, the Fail-safe GDT can solve
this problem, it is recommended to use between common mode and differential
mode with the voltage dependent resistor in series.